Reflector micromechanical structures are known, that are made, at least in part, with semiconductor materials and using MEMS technology.
These micromechanical structures may be integrated in portable electronic apparatuses, such as for example tablets, smartphones, PDAs, for optical applications, in particular for directing, with desired modalities, beams of light radiation generated by a light source, typically a laser. Thanks to the reduced dimensions, micromechanical structures enable stringent requirements to be met as regards occupation of space, in terms of area and thickness.
For instance, reflector micromechanical structures are used in miniaturized projector devices (the so-called picoprojectors), which are able to project desired images or patterns of light at a distance.
Reflector micromechanical structures generally include a mobile structure, which carries a reflecting element or mirror element (i.e. of a material having a high reflectivity for a particular wavelength, or band of wavelengths), made in a body of semiconductor material so as to be mobile, for example with a movement of inclination and/or rotation, for directing an incident light beam in a desired way by varying a direction of propagation thereof; and a supporting structure, which is also made starting from a body of semiconductor material, coupled to the mobile structure, having functions of supporting and handling. In the supporting structure, a cavity is generally formed, underneath and in a position corresponding to the mobile structure with its reflecting element, for enabling freedom of movement and rotation thereof.
Typically, the direction of propagation of the optical beam is varied in a periodic or quasi-periodic way for performing a scanning of a portion of space with the reflected optical beam. In particular, in reflector micromechanical structures of a resonant type, an actuation system causes oscillation of the reflecting element in a substantially periodic way about a resting position, the oscillation period being as close as possible to the resonance frequency in order to maximize the angular distance covered by the reflecting element during each oscillation and thus maximize the size of the scanning space portion.
Among reflector micromechanical structures, biaxial-actuation structures are known, in which the reflecting element is actuated with respect to two different mutually orthogonal actuation axes according to a so-called Lissajous scanning path.
FIG. 1 illustrates in a schematic and simplified manner a reflector micromechanical structure, designated as a whole by 1.
The reflector micromechanical structure 1 comprises: a frame 2, in the example shown having a square ring shape in a horizontal plane xy defined by a first horizontal axis x and a second horizontal axis y (and coinciding with the plane of a main surface 2A of the same frame 2); and a mobile mass 4, in the example shown having a circular shape in the horizontal plane xy.
The frame 2 defines, inside it, a window 5, in which the mobile mass 4 is housed, and is connected by first elastic elements 6 to an anchorage structure 7, external to the same frame 2, fixed with respect to a substrate 8 (represented schematically) of the body of semiconductor material in which the reflector micromechanical structure 1 is provided.
In particular, the first elastic elements 6 extend aligned along the first horizontal axis x, on opposite sides of the frame 2, connecting respective portions of the frame 2 to anchoring elements 7A, 7B of the anchorage structure 7 (that are, for example, in a way herein not illustrated, vertical pillars that connect to the aforesaid substrate 8).
The first elastic elements 6 are compliant to torsion for enabling a movement of rotation of the frame 2 with respect to the anchorage structure 7 and to the substrate 8, out of the horizontal plane xy and about the first horizontal axis x.
The mobile mass 4 carries, at the top, a mirror element 4′, of a material with high reflectivity for the light radiation to be reflected, such as, for example, aluminum or gold, and is connected to the frame 2 by second elastic elements 9, which extend aligned along the second horizontal axis y, on opposite sides of the mobile mass 4.
The second elastic elements 9 are compliant to torsion for enabling a movement of rotation of the mobile mass 4 with respect to the frame 2, out of the horizontal plane xy and about the second horizontal axis y; further, the second elastic elements 9 are rigid with respect to bending, so that the mobile mass 4 is rigidly coupled to the frame 2 in the movement of rotation about the first horizontal axis x.
The reflector micromechanical structure 1 further comprises: a first actuation structure 10 (represented schematically) coupled to the frame 2 and configured to cause an actuation movement of rotation of the same frame 2 about the first horizontal axis x, as a function of appropriate electrical driving signals such as to generate a twisting moment Tx about the first horizontal axis x; and a second actuation structure 11 (which is also represented schematically), coupled to the mobile mass 4 and configured to cause a respective actuation movement of rotation of the same mobile mass 4 about the second horizontal axis y, as a function of further electrical driving signals, such as to generate a respective twisting moment Ty about the aforesaid second horizontal axis y.
The first and second actuation structures 10, 11, as a function of the respective electrical driving signals, thus enable rotation of the mobile mass 4, and the associated mirror element 4′, about the first and second horizontal axes x, y, in this way enabling creation of a desired biaxial scanning pattern of the reflected light beam.
So far the following principles of operation have been proposed for the aforesaid first and second actuation structures 10, 11: electrostatic (in which respective sets of comb-fingered electrodes are coupled to the frame 2 and to the mobile mass 4, for generation of electrostatic attraction forces and generation of the aforesaid twisting moments Tx, Ty); piezoelectric (in which piezoelectric elements are mechanically coupled to the first and second elastic elements 6, 9 to cause torsion thereof and consequent generation of the twisting moments Tx, Ty); and electromagnetic (a coil is in this case arranged in a position corresponding to the frame 2 for generating, due to passage of an electric current, a magnetic field designed to generate the twisting moments Tx, Ty).
An example of a reflector micromechanical structure operating according to the electrostatic principle is, for example, described in U.S. Pat. No. 7,399,652 (incorporated by reference).
It has been noted that the solution described previously, irrespective of the principle of actuation used, is affected by certain limitations as regards the efficiency of actuation and the size of the resulting structure.
In particular, high values of the driving signals are generally to be supplied to the actuation structures to obtain the desired twisting moments Tx, Ty, for example high voltages of the order of 100 V for electrostatic actuation structures, or of 50 V for piezoelectric actuation structures, or high currents, for example of the order of 100 mA, in the case of electromagnetic actuation structures.
This problem is evidently more felt for biaxial-actuation micromechanical structures, where generally distinct driving circuits and actuation structures are required for each axis of actuation (with a consequent increase in the size and the manufacturing costs).
There is a need in the art to solve, at least in part, the above problems, which afflict micromechanical structures of a known type, and in particular to provide a structure with an improved actuation efficiency.